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OF RADIANT HEAT THROUGH DIFFERENT BODIES. 31 
The brown colour of the crystal was so decided that when it was laid 
on a printed page in which the letters were very large, and placed in 
the fullest light, even the traces of the letters could not be distinguished. 
The paper and the printed characters became confounded together and 
presented the same dark hue. This crystal, however, transmitted 19°, 
while the thin plate of alum transmitted only 6°. 
A body may then be very opake and afford a very easy passage to 
the rays of heat ; or very transparent and intercept the greatest part 
of them. It is therefore necessary to distinguish those bodies which 
possess a capacity for calorific transmission from those which possess a 
capacity for luminous transmission, by giving them different denomina- 
tions. The terms ¢ranscaloric and diathermanous* (transcaloriques ou 
diathermanes) seem to me to be best suited to this purpose, as being 
most analogous in form to the epithets ¢ransparent and diaphanous, 
applied to bodies endowed with the property of transmitting light. 
After the statement made in respect to the smoky rock crystal, one 
might be tempted to ask whether there are any transcaloric substances 
totally opake. To that question no answer can be given until the effect 
of calorific radiation upon all known bodies has been tried, and this I 
am far from having done. I can only go so far as to say that pyro- 
ligneous acid in the rough state, and Peruvian balsam, though almost 
completely opake, afford perceptible transmissions of radiant heat. But 
all the diathermanous substances that I have subjected to experi- 
ment are comprised within that class of bodies which possess some de- 
gree of transparepcy. Those kinds of metal, wood, and marble which 
totally obstruct the passage of light obstruct that of heat also. Some 
other bodies, such as carburet of sulphur, rock salt, and Iceland spar, 
allow both kinds of rays to pass at the same time. It is therefore pro- 
bable that calorific transmission cannot take place without a certain de- 
gree of transparency}; but it cannot take place abundantly without the 
cooperation of another quality, which varies as the bodies happen to be 
crystallized or without crystallization. We find, in fact, that in the dif- 
ferent sorts of glass and liquids it follows the order of the different de- 
grees of refrangibility ; for flint-glass possessing a greater refracting 
power than crown-glass affords an easier passage to the caloric radia- 
tion. Carburet of sulphur is at the same time more refracting and 
* The first of these terms requires no explanation. The second is derived 
from d:c, through, and Szgueaiva, to heat, as the word diaphanous is derived from 
dia, through, and Qaiva, to show. 
+ Ihave since found that the perfectly opake glass employed in the con- 
struction of mirrors designed to show the polarization of light transmits a con- 
siderable quantity of caloric rays. These obscure rays emerging from the dark 
glass may be employed in some curious experiments which we shall mention 
in the second Memoir. 
